Demagnetizing circuit

ABSTRACT

A circuit for selectively producing magnetized areas on a sheetlike magnetizable material utilizing a plurality of magnetic heads adjacent to the magnetizable material. A DC current is first passed through the coils in the heads and then a decreasing AC current is passed through selected coils. The decreasing AC current is produced by a radiant energy controlled resistance in series with the selected coils. A lamp controlled by an RC circuit illuminates the radiant energy controlled resistance to produce the decreasing AC current.

United States Patent Blitchington, Jr.

1451 Sept. 5, 1972 541 DEMAGNETIZING CIRCUIT 72 Inventor: Frank 11.Blitchington, Jr., Greensboro, NC.

[73] Assignee: Western Electric Company, Incorporated, New York, NY.

[22] Filed: Nov. 10, 1969 [21] Appl. No.: 876,637

[52] US. Cl. ..340/l74.l R, 179/1002 D, 315/8, 335/284 [51] Int. Cl..H0lj 29/06 [58] Field of Search ..307/3l1, 312; 179/1002 D; 340/174.1D, 174.1 G; 317/1575; 335/284;

[56] References Cited UNITED STATES PATENTS 3,218,396 11/1965 Mllllin;..179/1o0.2 K 3,218,620 11/1965 Clunis ..340/ 1.74.1 D

3,346,703 10/1967 Mullil'l er a1 ..179/1oo.2 K 3,450,933 6/1969 Haydenet a1 ..315/s 2,743,492 5/1956 Easton ..335/2s4 Primary Examiner-MaynardR. Wilbur Assistant Examiner-Jeremiah Glassman AttorneyW. M. Kain, R. P.Miller and B. l. Levine [57] ABSTRACT v A circuit for selectivelyproducing magnetized areas on a sheet-like magnetizable materialutilizing a plurality of magnetic heads adjacent to the magnetizablematerial. A DC current is first passed through the coils in the headsand then a decreasing AC current is passed through selected coils. Thedecreasing AC current is produced by a radiant energy controlledresistance in series with the selected coils. A lamp controlled by an RCcircuit illuminates the radiant energy controlled resistance to producethe decreasing AC current.

6 Claims, 5 Drawing Figures PATENTEDsEP 5:912

SHEET 2 BF 3 r w 5 M PATENTEDSEP m? 3.689.904

SHEEI 3 OF 3 1 DEMAGNETIZING cmcurr BACKGROUND OF THE INVENTION Thereare many prior art circuits for changing the magnetic state 'of apermanent magnetic object or material. Generally, 'a DC current appliedthrough a coil in close proximity to the object creates a magnetic fieldwhich magnetizes the object in a selected direction An objectis'dema'gnetized by applying a decreasing alternating magnetic field tothe object. The decreasing magnetic field in the prior art has beenproduced by supplying a decreasing current to a coil by means of amanual or motor driven potentiometer or variable transformer. Suchmechanical devices are unduly cumbersome and slow in operation.

SUMMARYIOF THE INVENTION magnetic object which produces a decreasingmagneticfield to demagnetize the object. A radiant energy emissiondevice which is energized by a decreasing current controlsrthe radiantenergy controlled resistance to produce a decreasing alternating currentiii the coil to demagnetize the magnetic object.

BRIEF DESCRIPTION OF THE DRAWINGS FIG.- 1 shows a magnetic card whichvis to be selectively magnetized in certain areas;

FIG. 2 shows a punched card placed on a reading device which controlsthe selective magnetization of the magnetic card shown in FIG. 1;

FIGS. 3 and 4 compose a diagram of a circuit embodying the principlesofthe present invention. FIG. 3 contains the left-hand side of thecircuit, while FIG. 4 contains the right-hand side of the circuit. Thetotal circuit may be reconstructed by placing FIGS. 3 and 4 side memorydevice side; and t FIG. 5 shows the operating times of various parts ofthe circuit shown in FIGS. 3 and 4.

DETAILED DESCRIPTION Referring first to FIG. 1, there is shown amagnetic card, 10 made of a sheet of aluminum'with a plurality ofpermanent magnetic objectsor elements 11-11, such as or 65 VICALLOY,embedded therein. The magnetic cards 10 are used in a magnetic device,such as is described in U.S. Pat. No. 3,163,855, issued on Dec. 29, 1964to A. H. Bobeck. The magnetic objects l1- 11 are selectively magnetizedor demagnetized to store or represent permanent information in themagnetic memoryto control the operation of an electronic telephonesystem. A plurality of writing or erasing heads 12-12 mounted on asuitable indexing carriage (not shown) over the card 10 are utilized tomagnetize and selectively demagnetize the objects 11-11. It has beenfound that magnetizing allthe objects ll-l1 in a i preferred directionand selectively demagnetizing produces fewer errors than demagnetizingall the elements 11-11 and selectively magnetizing. The AC magneticfield used to selectively demagnetize affects adjacent objectsl'ess thanthe DC magnetic field.

' Referring now to'FIGS. 3 and 4, there isshow'n an electrical circuitfor selectively energizing the heads 12-12 to produce the desiredmagnetization'on the card 10. FIG. 5 shows the operating times ofvarious components Of FIGS. 3 and 4 during one cycle of operation. Onecycle of operation is divided into two halves, the first half labeledmagnetize and the second half labeled demagnetize.

Referring back to FIGS. 3 and 4, there is shown a I plurality of coils31-38 each of which is associated with one of the heads 12-- 12 of themagnetic recording device. Contact arms 39-39 of a relay 40 engagenormally open contacts 27-27 to connect a first group of coils 31-34 allin series with resistors 28-28 when the relay 40 is not actuated. Whenthe relay 40 is actuated, the contact 'arms 39-39 engage normally opencontacts 29--29 to connect the second group of coils 3538 in series withthe resistors 28-28. Referring now to FIG. 5, the first half ormagnetizing portion of one cycle'is divided into a first portion orone-fourth cycle labeled 31-34 and asecond portion or second one-fourthcycle labeled 35-38. Also the second half or demagnetize portion isdivided into a third one-fourth cycle labeled 31-34 and a fourthone-fourth cycle labeled 35-38. Dividing the coils 31-38 into two groupsand switching between the two groups reduces the amount of circuitry. I

Alternating current is applied through a power plug 13 to.a transformer14 and a full wave rectifier 16. The output of the full wave rectifier16 on lines 17 and 18 is controlled by a voltage regulator 19 to producea +30 volt potential on line 17 and a l5 volt potential on line 18.Lines 17and 18 are connected to relays 21-26 which control the sequenceof operation of the circuit.

Referring to FIG. 2, there is shown a punched card 111 in a readingdevice 112. The presence of a properly oriented card 111 must be sensedby two photocells 107 and '109 to actuate a relay (FIG. 3) before acycle of operation may be initiated. A transistor 108 in series with thewinding of relay75 connected by a resistor to line 17 must be operatedby the photocell 109 located beneath the cutoff comer of the card 111.Also, a transistor 106 in parallel with the winding of the relay 75 mustremain unoperated by the photocell 107 beneath a non-cutoff corner ofthe card 111. A pair of lamps and 131 connected between lines 17 and 18provide illumination for the photocells 107 and 109. Operation of relay75 closes normally open contacts 74. If the card 111 is not oriented asshown in FIG. 2, the relay will not be actuated.

After the plug 13 is connected to a power source and before a cycle ofoperation has been initiated, a lamp 101 is illuminated by currentthrough a contact arm 102, a normally closed contact 103 of the relay 21and a normally closed contact 104 and a contact arm 73 of with anormally closed contact 94, acontact arm 93,

the winding'of relay 26 and contacts 74 to line 17. Actuation of therelay 26 connects the contact arm 93 to a normally open contact 95 whichis connected to ground throughout most of the cycle of operation by anormally closed contact, 96 and a contact arm 97 of the relay 25, anormally closed contact 98 and a contact arm 99 of.relay 21, or anormally closed contact 100 and a contact arm 66 of relay 23. One of therelays 21, 23 or 25 is unactuated during the cycle to maintain the relay26 actuated until the end of the cycle at which time the relays 21, 23and 25 are all actuated to deactuate the relay 26, as shown in FIG. 5.

- Actuation of the relay 26 engages the contact arm 73 with the contact72 to connect contact 72 to line 17 through the contacts 74. Much'of thecircuitry is connected to contact 72 and it should be remembered thatcontact 72 supplies a voltage of +30 volts until near the end of thecycle of operation. I

A primary winding of a transformer 51 is connected in series with apairof parallel connected radiant energy sensitive resistors 52 and 53across the power plug 13. A suitable type of radiant energy sensitiveresistor is LDR-25 power photocells sold by Delco Radio, Box 1018,Chestnut Station, Union, N.J., or type SLD4L photoconduc tive cells soldby Clairex Corporation, 1239 Broadway, New York, N.Y. A secondarywinding of the transformer 51 is connected through normally opencontacts 54 of a relay 55 to a first of the contact arms 39-39. Theradiant energy sensitive resistors 52 and 53 are energized by alamp 77in a light-tight enclosure 90 which is controlled by -a transistor 78which has its emitter and collector connected betweenthe lamp 77 and thecontact 72. A capacitor 80 and resistor 81 are connected in parallelfrom the base of the transistor 78 to ground. Current is applied to thebase of the through resistor 82 and 'diode 83. The capacitor 80 thendischarges through the resistor 81 and the based the transistor 78. Theparameters of the lamp 77, the

transistor 78, the capacitor 80, the resistor 81 and the resistor 82 areselected to cause the lamp to become slowly illuminated during theunactuated time of the relay 21 and to become slowly dark during theactuated time of the relafll. Thus, as shown in FIG. 5, the currentthroughthe radiation'sensitive resistors 52 and 53 slowly increasesduring each unactuated time of. relay 21 while the current through theradiation sensitive resistors 52 and 53 slowly decreases during eachactuated time of relay 21. The parameters of the resistors 81 and 82,the capacitor 80 and the transistor 78 may be selected to give anydesired change in currentthrough the radiation sensitive resistors 52and 53. For example, the current may be varied linearly, as shown inFIG. 5, or it may be varied exponentially.

The card reader 112 contains a plurality of photocells 114-123 for,reading the information punched in the 'card 111. A suitable typeofphotocell for use with the photocells 107, 109 and 114-123 is siliconreadout cell SSR sold by Solar Systems Inc. of

8241 Kimball Avenue, Skokie, Ill. The magnetic card writer hereindescribed is utilized to change magnetic information already stored inthe magnetic card 10. It

may be that only information stored in objects 11-11 beneath heads 12-12associated with the first group of coils31-34 is to be changed whilenochange is to be 'made in the objects '11-11 beneath the heads 12-12associated with the second group of coils -38. The

' photocells 114 and 119 sense holes in the card 111 which-indicatesthat changes are to be made in the objects associated with therespective first group of coils 31-34 and second group of coils 35-38.The photocells 114 and 119 operate a transistor 129 in series with therelay 55 which closes thenormally open contacts 54.

. The relay 55 is actuated only when there are changes transistor 78through a resistor 82, a diode 83, a norclosed contact 87 to ground. Acapacitor 89 is connected in parallel in the winding of the relay 21 tocause the relay 21 to operate as an oscillator bypassing current throughnormally closed contact 87 to build up charge on the capacitor 89 untilthe relay 21 is actuated at which time current is discharged from thecapacitor 89 through the winding of relay 21 until-the charge isinsufficient to maintain relay 21 actuated. As shown in FIG. 5, therelay 21 is actuated and deactuated four times during one cycle ofoperation. Referring back to FIGS. 3 and 4, a normally open contact 91connects the contact arm 88 through a contact arm 166 and a normallyopen contact 167 of the relay 25 to the start switch 92. If an operatorholds the switch 92 closed, the cycling of the circuit will be held upafter the relay 25 is actuated until the switch 92 is released.

When the relay 21 is actuated the contact arm 85 disengages the contact84 to interrupt the current flow being made in the respective first andsecond group of coils.

As shown in FIG. 5, the relays 24, 22, 25and 23 are actuated in astepping order. The relay 24 is actuated by current through a contactarm 139, a normally closed contact 140, a normally open contact 141 andcontact arm of the relay 21 connected to the contact 72 when the relay21 is first actuated. Relay 24 is maintained in actuated condition bycurrent through the contact arm 139, a normally open contact 142-, acontact arm 143 and either the normally closed contact 144 or thenormally open contact 145 of the relay 23 connected tothe contact 72.When the relay 23 is actuated, the contact arm 143' disengages thenormally closed contact 144 and engages the normally open contact 145.The time interval between the disengagement of the contact 144 and theengagement of the contact 145 is sufficient to cause deactuation of therelay 24.

The relay22 is actuated by current through a contact arm 147, a normallyclosed contact 148, a normally open contact 149 of therelay 24, acontact arm 150, the contact 84 and the contact arm 85 of the relay 21connected to the contact 72 when the relay 21 deactuates after the relay24 is actuated. Relay 22 is maintained actuated by current through thecontact arm 147, a normally open contact 158, a normally open contact156 and acontact arm 157 of the relay 24 connected to the contact 72.

The relay 25 is actuated by current passing through a contact arm 15 1,anormally closed contact 152, a normally open contact 1530f the relay22, a contact arm 154, the normally open contact 141 and the contact arm85 of the relay 21 connected to contact 72 of relay 26 when the relay 21isactuated after the relay 22 is actuated; Relay 25 is maintained in anactuatedcondition by current through the contact arm 151, a normallyopen contact 155, the normally open contact 156 and the contact arm 157of the relay 24 connected to 'contact 72 of-relay 26.

' The relay 23 is actuated by current through a contact arm 160, anormally closed contact 161, a normally open contact 162 of the relay25,'a contact arm 163, the normally closed contact 84 and the contactarm 85 of the relay 21 connected to the contact 72 when the relay 21 isdeactuated after the relay 25 is actuated. The relay 23 is maintained inactuated position by current through the contact arm '160-and a normallyopen contact 164 connected to the 'contact72.

The relays 22 and 25 are deactuated when the relay 24 is deactuated byactuation of the relay 23. As shown in FIG. 5, the actuation of relay 23and the deactuation of the relays 24, 22 and 25 occurs at the beginningof the second half or demagnetization portion of the cycle of operation.The relays 24, 22 and 25 are again actuated in a step wise fashion. Nearthe end of the complete cycle of operation, relays 21, .25 and 23 becomeall actuated to deactuate the relay 26 to disengage thecontact arm-73from the contact 72. Power from the line 17 through the contact 74 ismaintained on the contact 72 by a normally open contact 168 engaged by acontact arm 169 of relay 21.- When the relay 21 subsequently deactuates,.the line 17 is disconnected from the contact 72 to deactuate the relays24, 22, 25 and 23 as well as to remove the power from the other parts ofthe circuit connected to the contact 72.

During the magnetization portion of the cycle, the series circuit of thesecondary of the transformer 51', the contacts 54, the contact arms 39,the contacts 27, the coils 31-34 and the resistors 28 or the seriescircuit of the secondary of the transformer 51, the contacts 54, thecontact arms 39, the contacts 29, the coils 35-38 and the resistors 28are respectively connected to ground either through a diode 57 or adiode 58. Referring now to FIG. 1, there is shown a notch 59 in the card10 which is placed'over the normally open switch 63 FIG. 3) to controlthe direction of magnetization of the objects ll-l 1. The switch 61beneath the card at 62 is closed to connect the diode 57 to the circuitto rectify the current through the coils 3l-34'or the coils 35-39 toproduce a DC- current which magnetizes the objects 11-11 beneath thecoils 31-34 or the coils 35-39 in a selected direction during themagnetization portion of the cycle. Alternately the notch could belocated in the position 62 shown inphantom in FIG. 1,

the cycle, the relay 23 is actuated to connect ground through a contactarm 66 and a normally open contact 67 to the coils 31-34 or the coils 35-3 8 to shunt the switches 61 and 63 and diodes 57'and 58. Operation ofthe relay 23 also .connects power to relays 46-49 through a normallyopen contact 68, a contact arm 69, the normally-closed contact 70 or thenormally open contact 76 and the contact arm 71 to the contact 72.

The relays '46-49 control respective normally open 1 through theselected coils caused by contact resistance 7 of the respectivecontacts41-44.

A relay 125 in parallel with the relay 40 connected by a normally opencontact 176 and a contact arm 177 of the relay22 between thelines 17 and18 is deactuated during the first one-fourth of the cycle and the thirdone-fourth of the cycle -to connect a contact arm 126 to a normallyclosed contact 127 connected to the photocells 114-118 to read a firsthalf. of the holes punched in the card'lll. Similarly, during the secondone-fourth and the fourth one-fourth portion of the cycle of operation,the relay 125 is actuated toconnect the contact arm 126 to a normallyopen contact 128 connected to the photocells 119-123 to read the secondhalf of 'the card 111. Also, a lamp 132 connected througha normallyclosed contact 134 of the relay 22, acontact arm 135, a normally opencontact 136 of the relay 23 and a contact arm 137- across the secondaryof the transfonner 14 is illuminated during the third one-fourth portionof the-cycle to illuminate only the first half of the card 111.Similarly, a lamp 133 connected through a normally open contact 138 ofthe relay 22, the contact arm 135, the contact 136 of the relay 23 andthe contact arm 137 across the secondary of the transformer 14 toilluminate the second half of the card 111 during the fourth one-fourthportion of the cycle.

OPERATION Referring to FIGS. 3 and 4, initially the plug 13 is connectedto an AC power source. The magnetic card 10 is placed in a writingmechanism not shown) containing the heads 12-12 (FIG. 1) which haverespective coils 31-38. A switch 61 is closed beneath the area 62 whilea switch 63 is left open by the notch 59 to connect the first group ofcoils 31-34 in series with the diode 57, the contacts 54 and thesecondary of the transformer 51.

The punched card 111 FIG. 2) is properly inserted into' the card readerl12 .where the photocell 109' beneath the cutoff corner of the card 111is operated and the photocell 107 remains unoperated to actuate therelay and close the contacts 74 to illuminate the lamp 101. Thephotocell 114 senses a hole in the card 111 indicating that the firsthalf of the objects 11-11 beneath the heads l2-12 are to be changed. The

photocell 114 operates the transistor 129 which actuates relay 55 toclose contacts 54.

The operator presses and releases the switch 92 to actuate the relay 26to initiate the cycle of operation. The capacitor slowly charges tooperate the transistor 78 and slowly increases the radiation emitted bythe lamp The radiation sensitive resistors 52 and 53 slowly decreases inresistance to gradually increase ate relay 21.

When the relay 21 deactuates at .the end of the first one-fourth portionof the cycle, the relay 22 is actuated to actuate the relays 40 and125.The contact arms 39 of the relay 40 disengage contacts 27 andengageconthe diode S7. The contact arm 126 disengages contact 127 and engagescontact 128 to supply power to photocell 119. The photocell 119 senses ahole in the card 111 indicating that the second half of'the objects11-11 beneath the he'ads 12-12 areto be changed. The photocell operatesthe transistor 129 to actuate the relay 55 to close the contacts 54.

During the second one-fourthportion of the cycle, the capacitor 80 isagain slowly charged and discharged to produce a slow increase anddecrease of rectified current through the coils 35-38 to magnetizesecond half of the objects 11-11 in the selected direction. Also, thecapacitor 89 charges and discharges to actuate and deactuate the relay21.

At-the beginning of the third one-fourth portion of the cycle, the relay23 is actuated to short out the diode 57 toground. Power is applied bythe relay 23 to the relays 46-49 and the transistor pairs 171-174.Also,- the relay 22' is deactuated to deactuate the relays 40 and 125 toconnect the coils 3l-34and the photocells 114-118 back into the circuit.The photocells 115-118 in response to the presence of holes in the firsthalf of the card 111 which is illuminated by the lamp 132,

selectively operate the transistor pairs 171-174 to actuate the relays46-49 and close contacts 41-44. Thus, in

accordance with the information punched in the card 1 1 1 selected coils31-34 are shunted.

During the third one-fourth portion of the cycle, the current throughradiation sensitive resistors 52 and 53 is slowly increased anddecreased. Since the diode 57 is shunted, the current is AC currentwhich demagnetizes the objects 11-11 beneath the heads 12'-12 having thecoils 31-34 which are not shunted by the contacts It is to be understoodthat the above-described embodiments are simply illustrative of theprinciples of the invention and that many embodiments may be derivedwithout departing from the scope and spirit of the invention. Forexample, another arrangement other than the transistor 78, capacitor 80and resistors 81 and 82 may be used to vary the illumination of the lamp77.

What is claimed is:

1. A circuit for demagnetizing magnetic objects having remanent magneticstates comprising:

a coil to which the magnetic object may be placed in close proximity; aradiantenergy controlled resistance in series with the coil;

i I means for connecting the radiant energy sensitive resistance andcoil inseries with an alternating current source; 1 v

a radiant energy emission device in close proximity to the radiantenergy controlled resistance; and means for energizing "the radiantenergy emission device to produce a decreasing alternating current inthe coil to demagnetize the magnetic object.

2. A demagnetizing circuit as defined in claim 1, wherein the radiantenergy controlled resistance is sensitive to radiation in the visuallight range and the radiant energy emission device is a lamp and alight-tight enclosure contains both the radiant energy controlledresistance and the radiant energy emission device.

3. A demagnetizing circuit as defined in claim 1, wherein the energizingmeans includes a resistor and a capacitor and the discharge time of thecapacitor through the resistor controls the excitation of the radiantenergy emission device.

4. A circuit for demagnetizing magnetic objects comprising:

. a coil to which a magnetic object may be closely the coil, saidresistance being responsive to radia- I a tion in the infrared andvisual light spectrum; a lamp; v I l an enclosure impervious to infraredandvisual radiation for containing'the radiant energy controlled 5resistance and lamp; means for connecting the lamp to an energy source;a semiconductor device having its power electrodes connected in serieswith the energy source and the p; I a capacitor connected to the controlelectrode of the semiconductor device; and r a resistor connected to thecapacitor to discharge the capacitor such that the semiconductor devicebecomes slowly nonconduetive to cause the demagnetizing of the magneticobject. 5. A circuit for selectively storing bits of information on amagnetizable sheet-like material comprising ated to end the cycle andprepare the circuit for another cycle of operation.

a plurality of magnetic beads each having a coil arranged to extendacross the magnetizable material;

means for connecting the coils of the heads in series with a source ofalternating current; means for connecting a diode in series with theheads during the first portion of a cycle of operation such that directcurrent .passes through the coils of the heads to magnetize the materialadjacent to each resistance to slowly demagnetize selected areas adhead;a jacent to the magnetic heads. means for selectively shunting certainof the mag 6. A circuit as defined in'claim wherein the means neticheads; for producing a slowly decreasing quantity of energy ina radiantenergy controlled resistance in series with 5 dudes lamp, a capacitor aresistor and the h lg i current source; d discharge time of thecapacitor through the resistor means for producing a slowly decreasingquantity of controls the excltauon ofth? lamp energy to impinge on theradiant energycontrolled

1. A circuit for demagnetizing magnetic objects having remanent magneticstates comprising: a coil to which the magnetic object may be placed inclose proximity; a radiant energy controlled resistance in series withthe coil; means for connecting the radiant energy sensitive resistanceand coil in series with an alternating current source; a radiant energyemission device in close proximity to the radiant energy controlledresistance; and means for energizing the radiant energy emission deviceto produce a decreasing alternating current in the coil to demagnetizethe magnetic object.
 2. A demagnetizing circuit as defined in claim 1,wherein the radiant energy controlled resistance is sensitive toradiation in the visual light range and the radiant energy emissiondevice is a lamp and a light-tight enclosure contains both the radiantenergy controlled resistance and the radiant energy emission device. 3.A demagnetizing circuit as defined in claim 1, wherein the energizingmeans includes a resistor and a capacitor and the discharge time of thecapacitor through the resistor controls the excitation of the radiantenergy emission device.
 4. A circuit for demagnetizing magnetic objectscomprising: a coil to which a magnetic object may be closely placed;means for connecting the coil to a source of alternating current; aradiant energy controlled resistance in series with the coil, saidresistance being responsive to radiation in the infrared and visuallight spectrum; a lamp; an enclosure impervious to infrared and visualradiation for containing the radiant energy controlled resistance andlamp; means for connecting the lamp to an energy source; a semiconductordevice having its power electrodes connected in series with the energysource and the lamp; a capacitor connected to the control electrode ofthe semiconductor device; and a resistor connected to the capacitor todischarge the capacitor such that the semiconductor device becomesslowly nonconductive to cause the demagnetizing of the magnetic object.5. A circuit for selectively storing bits of information on amagnetizable sheet-like material comprising: a plurality of magneticheads each having a coil arranged to extend across the magnetizablematerial; means for connecting the coils of the heads in series with asource of alternating current; means for connecting a diode in serieswith the heads during the first portion of a cycle of operation suchthat direct current passeS through the coils of the heads to magnetizethe material adjacent to each head; means for selectively shuntingcertain of the magnetic heads; a radiant energy controlled resistance inseries with the alternating current source; and means for producing aslowly decreasing quantity of energy to impinge on the radiant energycontrolled resistance to slowly demagnetize selected areas adjacent tothe magnetic heads.
 6. A circuit as defined in claim 5, wherein themeans for producing a slowly decreasing quantity of energy includes alamp, a capacitor and resistor and the discharge time of the capacitorthrough the resistor controls the excitation of the lamp.